1. Field of the Invention
The present invention generally relates to radio communications systems to which Orthogonal Frequency Division Multiplexing (OFDM) is applied in downlink and specifically relates to user apparatuses and cell search methods.
2. Description of the Related Art
As a communications scheme to succeed W-CDMA (Wideband Code Division Multiple Access) and HSDPA, Long Term Evolution (LTE) is being studied in a W-CDMA standardization body called 3GPP. Moreover, as radio access schemes, OFDM is being considered for downlink, while SC-FDMA (Single-Carrier Frequency Division Multiple Access) is being considered for uplink (see 3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA”, June 2006, for example).
The OFDM, which is a scheme for dividing a frequency band into multiple narrow frequency bands (sub-carriers) and overlaying data onto the respective frequency bands for transmission, densely arranges the sub-carriers on the frequency axis such that one sub-carrier partially overlaps another sub-carrier without their interfering with each other, making it possible to achieve high-speed transmission and to improve frequency utilization efficiency.
The SC-FDMA is a transmission scheme which divides a frequency bandwidth and transmits using different frequency bands among multiple terminals to make it possible to reduce interference between the terminals. The SC-FDMA, which features a reduced variation in transmission power, makes it possible to achieve wide coverage as well as low power consumption of the terminals.
In the LTE, the OFDM provides for two types of CPs for reducing the effect of inter-symbol interference by a delay wave, namely a Long CP and a Short CP, each with a different length. For example, the Long CP is applied in a cell with a large radius and at the time of transmitting an MBMS (Multimedia Broadcast Multicast Service) signal, while the Short LP is applied in a cell with a small radius. The number of OFDM symbols within one slot is 6 when applying the Long CP and 7 when applying the short CP.
Now, in a radio communications system using the W-CDMA or the LTE, etc., a mobile station must generally detect a cell with a good radio quality for the own station based on a synchronization (sync) signal, etc., at the time of turning on the power, during camping, during communications, or at the time of a DRX reception during communications. The process, which is meant to search for a cell to which a radio link is to be connected, is called a cell search. The cell search method is generally determined based on a time needed for the cell search as well as a throughput of the mobile station at the time of conducting the cell search. In other words, the above-described cell search method must be a method such that the time required for the cell search is short and the throughput of the mobile station at the time of conducting the cell search is small.
In the W-CDMA, the cell search is conducted using two types of sync signals, namely a Primary SCH (P-SCH) and a Secondary SCH (S-SCH). Similarly, conducting the cell search using the two types of the sync signals of the P-SCH and S-SCH are also being considered in the LTE.
For example, as the cell search method, a cell search method is being considered such that the P-SCH with one sequence and S-SCH with multiple sequences are transmitted at a time interval of once every 5 ms (See R1-062990, Outcome of cell search drafting session, for example.). In the above-described method, the P-SCH specifies a downlink receive timing from each cell; while the S-SCH transmitted in the same slot specifies cell-specific information sets such as receive frame timing detection and cell or cell group ID. Here, it is generally possible to use a channel estimation value determined from the above-described P-SCH in demodulating and decoding the above-described S-SCH. Then, for grouping the cell IDs, the cell IDs to be grouped are thereafter detected form those cell IDs belonging to the group ID of the detected cell. For example, the cell ID is calculated based on a signal pattern of a pilot signal. Moreover, the cell ID is calculated based on the demodulation and decoding of the P-SCH and the S-SCH, for example. Alternatively, the cell ID may be included as an information element of the S-SCH without the cell ID grouping. In this case, the mobile station can detect the cell ID at the time of demodulating and decoding the S-SCH.
However, in an inter-station sync method in which signals from each cell are being synchronized, when the above-described cell search method is applied, the S-SCHs transmitted from multiple cells in different sequences are demodulated and decoded based on the channel estimation value which is determined from the P-SCHs transmitted from multiple cells in the same sequence. Thus, there is a problem of a transmission characteristic of the S-SCH being degraded. Here, the transmission characteristics also include a time needed for the cell search, for example. For an inter-station non-sync system in which signals from each cell are not being synchronized, receive timings of the P-SCH sequences transmitted from the multiple cells differ from one cell to another of the multiple cells. Thus, such a problem as described above does not occur.
In order to prevent a degradation in the S-SCH characteristics in the inter-station sync system, a cell search method is being considered such that the number of the P-SCH sequences are increased from 1 to a number no less than 2 (for example, 3 or 7) (See R1-062636, Cell Search Performance in Tightly Synchronized Network for E-UTRA, for example.). Alternatively, a method is being proposed for transmitting the P-SCH in different transmission intervals per cell in order to prevent degradation in characteristics of the S-SCH in the inter-station sync system (See R1-070428, Further analysis of initial cell search for Approach 1 and 2-single cell scenario, for example.). In the above-described method, the P-SCHs having different timings of receiving from the multiple cells may be used in the demodulating and decoding of the S-SCH. Thus, it is made possible to prevent the S-SCH characteristic degradation as described above.
Now, from a point of view of cell design, it is deemed that the larger the number of sequences of the P-SCH and the types of transmission intervals as described above, the better. This is because, the smaller the number of sequences of the P-SCH or the types of transmission intervals, the higher a probability of the P-SCH sequences in neighboring cells becoming the same, or the higher a probability of the P-SCH transmission intervals becoming the same, so that a probability of occurrence of the S-SCH characteristic degradation in the inter-station sync system becomes higher.
Moreover, there is a tradeoff relationship between the time needed for conducting the cell search as described above, or the transmission characteristics of the cell search, and the throughput of the mobile station when the cell search is conducted. Thus, it is desirable to be able to select whether the transmission characteristics of the cell search is to be emphasized or the throughput of the mobile station when the cell search is conducted is to be emphasized.